It is a closed system. No voting machine is every connected to the internet. Heck on election day, Harris County still uses paper poll books for voting. Each booth sends its vote to a machine at the site (the Judge Booth Controllers) which is connected by a cable to the voting booths. This is a closed system. The machines could be reprogrammed in the election office but there would be signs of tampering. Some of the testing done before each election looks for signs of such tampering. Once certified and after the seal is put on, there is no way to tamper with a machine when it is in the fields
Part of the closing of poll procedures is to verify the number of votes cast on the JBC and compared that to the number of voters who voted. In addition, a report is generated when the polls are opened to show that there are no votes on the machine (this is called a zero tape).
I work with the Harris County party. They have teams of people who watch the testing and verify the status of the machines prior to each election. Harris County also has people who watch the county cut the seals on each machine and watch the removal and counting of each vote. My friend who was in charge was at the election office until 2 in the morning on election day in 2016 even though the races in Harris County were mainly decided by early vote.
The issues with the current machines are that they are old and are breaking down. We have two JBCs at most voting locations to handle this issue. I was nice and agreed to serve as the GOP judge in the 2014 primary and my machines broke down a couple of times and had to be rebooted (you unplug and plug back in each voting both in order). I had a great college student with me and we got good at this.
Most counties are looking for new machines. Travis County spent a couple of years looking for a new design and eventually gave up https://www.austinchronicle.com/news/2017-10-13/county-ditches-star-votes-innovative-voting-system/
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